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_Chemical Reactions _
By: I Don't Know
Chemical reactions Essay submitted by Unknown Chemical reactions are the heart
of chemistry. People have always known that they exist. The Ancient Greeks
were the firsts to speculate on the composition of matter. They thought that
it was possible that individual particles made up matter. Later, in the
Seventeenth Century, a German chemist named Georg Ernst Stahl was the first to
postulate on chemical reaction, specifically, combustion. He said that a
substance called phlogiston escaped into the air from all substances during
combustion. He explained that a burning candle would go out if a candle
snuffer was put over it because the air inside the snuffer became saturated
with phlogiston. According to his ideas, wood is made up of phlogiston and
ash, because only ash is left after combustion. His ideas soon came upon some
contradiction. When metal is burned, its ash has a greater mass than the
original substance. Stahl tried to cover himself by saying that phlogiston
will take away from a substance's mass or that it had a negative mass, which
contradicted his original theories. In the Eighteenth Century Antoine-Laurent
Lavoisier, in France, discovered an important detail in the understanding of
the chemical reaction combustion, oxigine (oxygen). He said that combustion
was a chemical reaction involving oxygen and another combustible substance,
such as wood. John Dalton, in the early Nineteenth Century, discovered the
atom. It gave way to the idea that a chemical reaction was actually the
rearrangement of groups of atoms called molecules. Dalton also said that the
appearance and disappearance of properties meant that the atomic composition
dictated the appearance of different properties. He also came up with idea
that a molecule of one substance is exactly the same as any other molecule of
the same substance. People like Joseph-Lois Gay-Lussac added to Dalton's
concepts with the postulate that the volumes of gasses that react with each
other are related (14 grams of nitrogen reacted with exactly three grams of
hydrogen, eight grams of oxygen reacted to exactly one gram of hydrogen, etc.)
Amedeo Avogadro also added to the understanding of chemical reactions. He said
that all gasses at the same pressure, volume and temperature contain the same
number of particles. This idea took a long time to be accepted. His ideas lead
to the subscripts used in the formulas for gasses. From the work of these and
many other chemists, we now have a mostly complete knowledge of chemical
reactions. There are now many classification systems to classify the different
types of reactions. These include decomposition, polymerization, chain
reactions, substitute reactions, elimination reactions, addition reactions,
ionic reactions, and oxidation-reduction reactions. Decomposition reactions
are reactions in which a substance breaks into smaller parts. As an example,
ammonium carbonate will decompose into ammonia, carbon dioxide, and water.
Polymerization reactions are reactions in which simpler substances combine to
form a complex substance. The thing that makes this reaction unusual is that
the final product is composed of hundreds of the simpler reagent (a substance
that contributes to a chemical reaction) species. One example is the
polymerization of terephthalic acid with ethylene glycol to form the polymer
called Dacron, a fibre, or Mylar, in sheet form: nH2OC(C6H4)CO2H + nHOCH2CH2OH
-* [...OC(C6H4)CO2CH2CH2O...]n 2nH2O in which n is a large number of moles. A
chain reaction is a series of smaller reactions in which the previous reaction
forms a reagent for the next reaction. The synthesis of hydrogen bromide is a
good example: H2 + Br2 -* 2HBr This is a simple equation that doesn't properly
prove the reaction. It is very complex and starts with this: Br2 -* 2Br The
next three reactions are related and should be grouped together. A substation
reaction is a reaction in which a substance loses one or more atoms and
replaces them with the same number of atoms of another element from another
substance. Here is the example of chloroform that reacts with antimony
triflouride: CHCl3 + SbF3 -* CHClF2 An elimination reaction is a reaction in
which a compound is broken into smaller parts when heated. Here is an example
when the same substance is heated and goes through another reaction: 2CHClF2
-* C2F4 + 2HCl An addition reaction is a reaction in which atoms are added to
a molecule. If the added atoms are hydrogens, then the reaction is called a
hydrogenization reaction. If Oleic acid is hydrogenized, this what you get:
C18H34O2 + H2 -* C18H36O2 Another reaction is called an ionic reaction. It
occurs between two ions and can happen very quickly. For example, when silver
nitrate and sodium chloride are mixed you get silver chloride: AgNO3 + NaCl -*
AgCl + NaNO3 The last type of reaction is called oxidation-reduction. These
are reactions that involve a change in oxidation number. It is a reaction if
the oxidation number goes up. It is a reduction reaction if the oxidation
number goes down. It is now known that there are three types of chemical
reactions. They are classified into three types: exoergic (exothermic),
endoergic (endothermic), and aergic (athermic). In these cases, energy is
supplied, but the different types of reactions initiate the energy
differently. Exoergic, or exothermic, reactions release energy during the
reaction. Combustion is one of the major reactions that do this. The burning
of wood, or any other fuel, gives off heat, and the burning of glucose in our
bodies gives off both energy and heat. Endoergic, or endothermic, reactions
absorb energy during the reaction. The melting of an ice cube is an example of
an endothermic reaction. Aergic, or athermic, reactions neither give off nor
absorb energy. There are very few cases in which this happens. There are some
things that must be considered in a chemical reaction. Kinetics is one of
these things. Kinetics decides The speed of the reaction and what is happening
on a molecular level. There are a few things that decide the course and speed
of the reaction. The first thing is the reactants. Different reactants react
at different speeds. Even the position of the reactants will affect the
reaction rate. The next thing is the catalyst that contributes a needed
substance to the reaction. It Is part of the energy considerations. The
catalyst is an outside substance that is included in the reaction, but is not
consumed during the reaction like the reactants are. They cannot make
impossible reactions occur, they only contribute to the reaction to increase
the reaction rate. There are also such things as negative catalysts, or
inhibitors. Inhibitors retard the reaction rate. This is also a way to control
reactions. A good example in nature of a catalyst is in a firefly. The
reaction that releases the light is complex. Lucifern, which the firefly makes
naturally, is oxidized in the presence of luciferase, another natural enzyme,
which acts as a catalyst in the reaction. Thus, the reaction makes an excited
form of luciferase, which soon returns to its original state. Energy as light
is released when the lucifrase returns to its normal state. The insect can
easily control this reaction with an inhibitor it naturally makes. Another
contributor in this consideration is entropy. It is the measure of energy not
available for work in the reaction that becomes energy moved to disorder.
Entropy is simply a measurement of unusable energy in a closed thermodynamic
system. An acid and base reaction is another thing to consider. Acids and
bases react very readily to each other. When an acid and a base react, they
form water and a salt. Acids and bases neutralize each other and form a salt
as a byproduct. This reaction reaches what is called equilibria, (When a
substance is completely neutral in charge and acidity). One example of how
acids and bases react is the reaction of calcium hydroxide and phosphoric acid
to produce calcium phosphate and water: 3Ca(OH)2 + 2H3PO4 -* Ca3(PO4)2 + 6H2O
The last detail is the reaction conditions. The temperature, humidity, and
barometric pressure will affect the reaction. Even a slight change in any one
of these could change the reaction. There are many branches of Chemistry that
use chemical reactions, infact, almost all of them. Here are some examples.
Photochemistry is one branch of chemistry that deals with chemical reactions.
It has to do with the radiant energy of all kinds formed during chemical
reactions. Photochemists will experiment with chemical reactions. They will
perform reactions normally only possible at high temperatures in room
temperature under ultra-violet radiation. The reaction rate can be controlled
for observation by varying the intensity of the radiation. X-rays and gamma
rays are commonly used in these procedures. The most important photochemical
reaction is photosynthesis. Carbon-dioxide and water combine with chlorophyll
as a catalyst to give off oxygen. Photochemical reactions are caused by
photons that are given off by the light source. The reactant molecules absorb
the photons and get excited. They are at such an excited state, they can
decompose, ionize, cause a reaction with other molecules, or give off heat.
Another science that uses chemical reactions is Biochemistry. They use them to
produce products that a person either can't produce or cannot do as well as
they should. The best example of this the production of insulin. It was first
produced in very tiny beads until someone realized that the body does in a
very similar way. The person was Robert B. Merrifeild. He was the first to
urge scientists to study living systems for the answers to problems that could
be solved with synthesizing chemical reactions in the body. This was actually
the first step toward the development of bionics. Scientists today are still
toying with chemical reactions. They are trying to control them with lasers.
Scientists are trying to use lasers to prod a chemical reaction that could go
one way or another, the way they want it to. They want to direct the molecules
in one direction. The control of photons to excite molecules and cause
reactions has been elusive. Recently, though, chemist Robert J. Gordon at the
University of Illinois achieved "coherent phase control of hydrogen disulfide
molecules by firing ultraviolet lasers of different wavelengths at them."
Laser chemistry looks promising and is a way that chemistry is still being
expanded. Again, chemical reactions are the main part of a branch of
chemistry. Here again, scientists are playing with chemical reactions. In
April of 1995, a chemist named Peter Schultz and a physicist named Paul McEuen
of the University of California at Berkly announced that they could control
chemical reactions molecule by molecule. "The key to the technique is to put a
dab of platinum on the microscopic tip of an atomic force microscope. (The tip
of such a microscope is a tiny cantilever that rides like a phonograph needle
just above the surface of a sample and reacts to forces exerted by the
electrons beneath it.)" The Platinum acts like a catalyst, stimulating a
reaction between two reactants, just stimulating a reaction one molecule at a
time. The molecules are stimulated in a pattern giving the wanted results.
This discovery opens doors for nanoengineering and material sciences. It gives
a good view of what happens, one molecule at a time. Chemical reactions are a
large part of chemistry. This paper is an overveiw of that extensive subject.
It gives a good idea about the history of chemical reactions as well as the
future. Hopefully, there will be no end to the expansion of chemistry and our
knowledge. Since Scientists are still experimenting, chemical reactions will
always be a part of chemistry.
_Bibliography _
Bibliography "Chemical Reactions," Encyclopedia Brittanica MACROPEDIA, 1995,
Vol. 15 "Dances With molecules," Science News, Vol. 147, May 27, 1995 Eastman,
Richard H., General Chemistry: Experimental and Theory, Holt, Rhinehart, and
Winston Inc., 1970 "One Molecule at a Time", Discover, January 1996 Pauling,
Linus and Peter,
Chemistryhttp://click.linksynergy.com/fs-bin/stat?id=pcZ8g7DjAzA=6424=2=0=http%
253A//search.borders.com/fcgi-bin/db2www/search/search.d2w/Details%253F%2526med
iaType%253DBook%2526prodID%253D2849598, W. H. Freeman and Co., 1975
"Reactions, Chemical," Encyclopedia Americana, 1982, Vol. 23 "Reactions,
Chemical," Academic American Encyclopedia, 1991, Vol. 16
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